It is well known that ships, barges and other floating platforms roll, pitch and heave at sea and that such motion is undesirable in many fields. For example, such motion may be particularly undesirable when loading and unloading to and from the vessel. This is particularly the case for vessels involved with the offshore oil and gas industries. In that application it is common to unload and load, from and to a stationary structure e.g. a deck supported on a jacket on the sea bed or from and to another vessel.
Additionally, in the field of offshore gas and oil, the vessels may be extremely large so that, whilst the movement of the vessel is not very great when expressed in degrees of inclination, the movement at deck height is considerable, causing difficulties even in relatively calm conditions.
There are many known systems which aim to reduce roll and/or pitch motion of floating vessels. There are some systems that have been designed for relatively small vessels. For example, GB 2219973 describes a vessel in the hull of which there is a passageway which allows the free flow of water through it. As the passageway fills and drains, the natural period of the pitching/rolling motion is increased and the motion response of the vessel is reduced. In an improvement on this arrangement, such a tank may be connected to a pump so that the filling and draining of the tank can be controlled at least partially. However, such systerns are integral with the vessel itself and are difficult to install and costly and are not able to be easily transferred from one vessel to another.
Another system which aims to reduce instability of a larger vessel is described in U.S. Pat. No. 5,787,832. In that system, stabilizer assemblies are attached to the hull of the vessel. Each assembly includes an outrigger arm and a float arm which has a float attached to one end. The floats are in contact with the water surface at all times and the system works by increasing the effective width of the vessel so as to increase the natural period of its rolling/pitching motion.
Each stabilizer assembly has to be attached to the vessel through a very strong fastening that has to bear very high loads. U.S. Pat. No. 3,407,766 describes another system which aims to reduce the instability of a larger vessel by providing a stabilizing body below the vessel and connecting it by rigid struts such as steel I-beams which are able to transmit a force moment back to the vessel. A major drawback to an arrangement of this kind is the very considerable strength required of the struts in order to transmit force moment from the stabilizing body to the vessel.